In a vehicle transmission, it is often desirable to have an auxiliary transmission interposed between the primary transmission and the drive shaft of the vehicle. Such auxiliary transmission will normally provide two gear ranges for the vehicle. For example, in a small truck, it may be desirable to provide a higher than normal drive gear range to maximize gas mileage for highway driving. In a jeep or the like, a lower than normal drive gear range may be convenient for off-road operation.
An important design criteria for such auxiliary transmission is size. The transmission must be small so that it will fit in the vehicle in addition to the principal transmission and will not add excessive weight to the vehicle or unduly reduce other compartment sizes in the vehicle. In many cases, the verticle dimension or height of the auxiliary transmission is one critical size dimension because this dimension is limited below by the required ground clearance and above by limitations on intruding into the engine, primary transmission, and passenger space.
In the present invention, the size and weight criteria for an auxiliary transmission are met by the provision of a transmission having multiple countershafts. In accordance with one form of the invention, an auxiliary power transmission has first and second power shafts with first and second sun gears for being rotatably interconnected with the power shafts. The first and second sun gears have differing diameters, and a shift mechanism mechanically interconnects the first and second power shafts and shifts between engagement with the first sun gear and the second sun gear. A transmission housing is dimensioned to house and support the auxiliary power transmission, and first and second main bearings are provided in the housing for rotatably supporting the first and second power shafts, respectively. There are at least three first planet gears disposed about and continuously intermeshed with the first sun gears, and three second planet gears are disposed about and continuously intermeshed with the second sun gear. Three countershafts are provided with each shaft carrying one of the first planet gears on one of its ends and carrying one of the second planet gears on the other of its ends so that each of the three countershafts and the carried first and second planet gears form at least three planet gears sets that transmit power between the first and second sun gears. A plurality of countershaft bearings in the housing rotatably support the countershafts and the planet gear sets are positioned to entrap and at least partially support the first and second sun gears so that the countershaft bearings help support the first and second power shafts. The first and second main bearings are dimensioned according to the support of the first and second power shafts provided by the countershaft bearings. The planet gears sets are unequally spaced apart, one from the other, about the first and second sun gears to form at least one space between the planet gear sets that is large relative to at least one of the other spaces between the planet gear sets.
The large spacing between the planet gear sets provides a good access to the interior of the auxiliary transmission. In most cases, a shift mechanism will be located within the planet gear sets, and the large spacing will allow a shift fork to have easy access to the shift mechanism. Thus, the unequal spacing of the planet gear sets facilitates the shifting of the transmission.
Also, since the planet gear sets are unequally spaced, the housing may also be constructed with one or more flat sides which occurs at the larger spacing between the planet gear sets. One transverse dimension of the auxiliary transmission is, thus, reduced and the transmission may be mounted so that this reduced dimension is oriented to provide a minimized height for the auxiliary transmission. Since the height of the auxiliary transmission is often a critical dimension for such transmissions, the unequal spacing of the planet gear sets on the present invention provides a structure particularly suited for this type of transmission.
An additional advantage to unequal spacing is the total number of available gear ratios when not restricted to the conventional requirements for assembly of equally spaced planets, i.e., this non-conventional establishment of angular position of planets allows the use of any tooth combination of suns and planets and therefore just as many overall ratios to select from.
As used herein, the terms sun gear and planet gear are used in a broad descriptive sense to mean that the planet gears are positioned around the sun gears. By use of the term planet gear, it is not intended to imply that the planet gears must rotate about the sun gear and it is not meant to imply that the planet gears must be mounted in some type of carrier that could allow rotation of the planet gears about the sun gears. In the embodiment of the invention described herein, the planet gears are part of a countershaft set whose position (axis of rotation) is fixed with respect to the position (axis of rotation) of the sun gears.
In accordance with another aspect of the invention, it is important that the planet gear sets be located at alignment positions relative to the other planet gear sets in order to provide load sharing between countershafts in the auxiliary transmission. An alignment position is an angular position on the input and output sun gears where the teeth of the two sun gears may be aligned. By definition, a tooth of one sun gear at an alignment position will be aligned with a tooth of the other sun gear at least once during each revolution of the planet gears. When teeth are aligned, a plane passing through the center of a tooth and the center of one sun gear will pass through the center of a tooth on the other sun gear.
Also, in order to provide load sharing in the auxiliary transmission of the present invention, it has been found that the planet gear set should be machined from a single piece of stock with the two planet gears of the planet gear set having at least one pair of aligned tooth spaces. That is, at least one tooth space of one planet gear is aligned with a tooth space on the other planet gear in the set. Each aligned tooth space on one planet gear of a planet gear set must be marked as being aligned with a tooth space on the other planet gear and preferably the tooth alignment is held to within a tolerance of 0.0002 inches. In this construction, with the planet gear sets being machined from a single piece of stock and with the planet gear sets being carefully located, relative to one another, at alignment positions, a strong, smooth operating auxiliary transmission is achieved.